As President Reagan leaves today on the first leg of his journey to Moscow, administration officials are conspicuously playing down expectations that the summit meeting will be a climax of achievement marked by dramatic new agreements. The message is that the summit will represent but one more step in the steady, persistent process of improving US-Soviet relations.

``We will be pushing for a continued dialogue, for continued work throughout the remainder of this administration,'' says Rozanne Ridgway, assistant secretary of state for European affairs. ``We've asked ourselves how best to describe what this is and is not, and we all agree it is not a high watermark.''

While the May 29-June 2 summit is being portrayed as almost routine, Reagan administration officials acknowledge the occasion will be a television spectacular, revolving around the President's public appearances.

Mr. Reagan's schedule, in addition to five official meetings with Soviet General Secretary Mikhail Gorbachev, includes a host of people-to-people events that will enable Reagan to focus on human rights and promote his own brand of glasnost (openness).

The first US president to visit the Soviet Union in 14 years, Reagan clearly hopes to encourage the forces of change emerging there under Mr. Gorbachev's leadership.

``We do not expect a quick, radical transformation of the Soviet system,'' Reagan said in an address beamed to Europe yesterday. While the Soviets have made progress on human rights issues, he said, ``the human rights situation in the Soviet Union and Eastern Europe is far from good.''

Almost his first act will be to visit Danilov Monastery in Moscow, where he will meet with Russian Orthodox monks and view Russian icons in a symbolic gesture of support for religious practice in the Soviet Union.

Ukrainian Catholics have opposed the visit as appearing to sanction Moscow's control of the Russian church. But Reagan views the visit as an opportunity to stress the importance of religious freedom.

The substantive summit meetings will deal with the broad four-part agenda that the Reagan administration fashioned in 1985 and has pursued ever since: arms control, regional problems, human rights, and bilateral issues. Working groups will be discussing these questions, say US officials, and some issues may be referred to the two leaders for further progress.

Arms control. It is hoped the signed INF (Intermediate-Range Nuclear Forces) Treaty will be the centerpiece of the summit. If the Senate ratifies the pact in time, the two leaders will exchange formal certificates of ratification.

Not enough progress was made to conclude a broader strategic or long-range nuclear arms reduction treaty (START) by the time of the summit. But the two sides will continue to work on verification and other problems and possibly record progress made to keep the momentum of the negotiations going. Both Gorbachev and Reagan continue to voice hope that a START agreement can be completed this year.

The two leaders may also sign some minor arms agreements, including a ``confidence building measure'' that would in effect be one element of a strategic-arms accord. This would require prior notification of ballistic missile tests conducted on land and over water. A new verification protocol to the unratified 1974 treaty on peaceful nuclear explosions may also be signed as well as an agreement on how to conduct experiments at the US and Soviet nuclear test sites.

The two leaders are expected to review the issue of conventional arms and chemical weapons. But these are technically the subject of multilateral negotiations in Vienna and Geneva.

Regional issues. Afghanistan will still be prominent in the discussions. The US will encourage the Soviets to continue their withdrawal, say US officials, and will also talk about organizing an international program to help returning Afghan refugees.

But the focus will shift to other issues, above all the intensifying diplomatic efforts now under way to resolve the dispute in southern Africa.

``It seems that perhaps southern Africa is the regional issue that offers some prospect for ... moving the dialogue ahead,'' Secretary Ridgway told reporters last week. The US is pressing for a settlement that would include the removal of Cuban and other foreign forces from the area, independence for Namibia, and national reconciliation in Angola.

Moscow will be urged to exert its influence on Angola and Cuba to set a prompt timetable for the withdrawal of Cuban troops.

Washington would also like to get Soviet cooperation with its peace initiative in the Middle East but does not see much likelihood of this. ``There have been some indications of movement in the Soviet position,'' says Michael Armacost, under secretary of state. ``But I'd have to say that when we get down to the details, some things like a conference and representation, we're still quite a long way apart on those issues.''

Central America, the Persian Gulf, the Korean Peninsula, Cambodia, and Ethiopia are other regional issues expected to be discussed.

Bilateral issues. Agreements may be signed between the US Coast Guard and Soviet maritime authorities on search and rescue and ocean pollution. A comprehensive fishing treaty as well as a new three-year cultural agreement may be ready for signature, as well as agreements in the area of transportation and basic scientific research.

Human rights. Primary US attention will be given to the release of political prisoners, emigration, and the issue of religious freedom.

Reagan officials stress the President's desire for personal contacts with Soviets citizens. ``I think it's symbolic, but it's also the President's genuine desire ... really to listen to people, to hear their concerns, and to learn,'' says a senior administration official.